Prior apparatus for indicating the layer number of a web material wound on a reel include mechanisms such as a mechanical counter, geared to the reel for directly monitoring the turn number of the reel. However, mechanical counters produce mechanical loading on the reel, are inaccurate, and tend to wear out.
Prior attempts have been made to provide an indication of quantity, as opposed to layer number, of a web material wound on a reel without mechanically contacting the reel. In U.S. Pat. No. 3,564,219 to Metzeger, there is disclosed an electronic indicator for indicating the amount of a web material would upon a spool by generating a first train of pulses as a function of the rotational velocity of the reel, and a second train of pulses as a function of the linear velocity of the web material. As the web is wound onto or unwound from the reel, the rate at which the reel rotates varies in proportion to the amount of the web material wound thereon. Accordingly, by monitoring the number of web responsive pulses relative to the reel responsive pulses, an indication of the radius of the total web material accumulated on the reel can be ascertained. The actual layer number of web material wound on the reel, however, is not available. This is because the output of the Mutziger device is a function of only the radius of web accumulated on the reel. It can detect what proportion of the reel contains web, e.g., that the reel is 50 percent full, but not the actual number of layers of the web wound thereon. The actual layer number of the web can be derived only by measuring (a) the circumference of the outermost layer of web on a full reel, (b) the circumference of the reel hub, (c) the length of tape corresponding to a full reel, and (d) the circumference of the unknown layer and then numerically interpolating the circumference of the unknown layer to derive the layer number of the unknown layer of web wound on the reel. The numerical interpolation requires mental steps or complex electronic circuitry and is impractical for low cost, automatic systems.
As another disadvantage, the Mutziger device, providing only data indicative of amount of web wound on a reel, rather than layer number, is unsuitable for data addressing wherein data carrying web, such as magnetic tape, is first unwound from a storage reel to a predetermined layer of the web and then to a predetermined position within the layer.